1. Field of the Invention
This invention relates to an optical head unit for recording information data on an optical disk member, and a method of manufacturing the optical head unit.
2. Description of the Related Art
An optical head unit for use in recording information on an optical disk serving as a recording medium and reproducing information from the optical disk includes an actuator (movable unit) including an objective lens disposed near a recording surface of the optical disk, and a body unit (stationary unit) having a semiconductor laser element as a light source and a photodetector for converting a reflection laser beam received by the objective lens into an electric signal.
The objective lens is used to radiate onto the recording surface of the optical disk a laser beam of a predetermined wavelength produced by the semiconductor laser element and to receive the reflection laser beam reflected from the recording surface of the optical disk and to guide the reflection laser beam into the photodetector.
The photodetector photoelectrically converts the reflection laser beam received by the objective lens from the recording surface of the optical disk and produces an electric signal corresponding to the light intensity of the reflection laser beam.
Various methods for increasing the recording density have been proposed. For example, the diameter of a laser beam radiated on the recording surface of the optical disk by means of the optical head unit is reduced, i.e. the size of a recording pit is reduced; the interval of recording pits on the track on the optical disk is reduced; or the sensitivity of the photodetector for sensing the reflection laser beam from the optical disk is enhanced.
More than ten years have passed since the compact disks for musical use were developed. Moreover, various kinds of optical disks, typically CD-ROMs for personal computers or photo-CDs, have been developed for business use or household use.
The optical disks are mass-produced according to the standards stipulated in the industrial world. The standards themselves, however, have been altered to meet the demands of the times, for example, an increase in recording capacity or in number of functions (e.g. not only reproducing but also recording and/or erasing is performed). In addition, a number of new standards have been proposed.
As a result, about ten kinds of optical disks have been marketed.
It is practically not possible to prepare all types of drives matching with various optical disks, because of economic reasons, difficulty in handling the drives, or limited space for installation.
Under the circumstances, there is a demand for a method and an apparatus for reproducing information from different types of optical disks by means of a single drive. Needless to say, there is a similar demand for a method and an apparatus for recording and/or erasing information by means of a single drive.
According to one proposal, in order to provide different light spots of a laser beam necessary for reproducing or recording/erasing information on two or more kinds of optical disks with different recording densities, the actuator is provided with a necessary number of objective lenses with different numerical apertures and the optical objective lens is selected according to the kind of the optical disk.
However, in the optical head unit in which the objective lenses with different numerical apertures are switched, the tracking is controlled in the state in which the objective lens is shifted in the tracking direction (i.e. radial direction of the optical disk) relative to the parallel beam incident on the objective lens. In principle, while the apparatus is being operated, a beam shift (displacement between the center of intensity of the parallel beam and the center of optical axis of the objective lens) occurs more or less at all times.
According to another proposed method, when information recorded on an outer peripheral area of an optical disk needs to be reproduced while information on an inner peripheral area is being reproduced, the optical head unit is shifted in the radial direction of the optical disk to access a desired track in the state in which and the objective lens is shifted in the tracking direction by a predetermined amount.
In this method, the objective lens is restored at last to the shift-free state from the state in which it is shifted in the tracking direction. During the access, a tracking error signal having a predetermined quality level must be obtained to exactly seek a target track, though the objective lens is in the shifted state.
It is well known, however, that to shift a beam results in an offset of the tracking error signal or a decrease in signal amplitude. Consequently, the quality of the tracking error signal is degraded. In fact, there is a problem in that a beam is shifted due to machining precision of individual parts of the optical head unit and/or attachment precision of the parts.
To shift the beam, therefore, causes an offset in a focus error signal or a tracking error signal, or degrades C/N in reading recorded information signals.
In addition, the degree of shift of the beam differs among switchable objective lenses.
As a result, although the objective lenses with higher numerical apertures are used for optical disks on which information is recorded at high density, the recording/reproducing performance of the optical head unit is degraded.